docs: def match_variables(self, pattern, return_type='name'):
          ''' Return columns whose names match the provided regex pattern.
  
          Args:
              pattern (str): A regex pattern to match all variable names against.
              return_type (str): What to return. Must be one of:
                  'name': Returns a list of names of matching variables.
                  'variable': Returns a list of Variable objects whose names
                  match.
          '''
          pattern = re.compile(pattern)
          vars_ = [v for v in self.variables.values() if pattern.search(v.name)]
          return vars_ if return_type.startswith('var') \
              else [v.name for v in vars_]